to O3, 2016 brought a small improvement to 35% of the days. And again in 2017, 35% of the days were recorded as "bad" due to O3 levels. At times during the past two years, ozone concentration levels in the city reached such extreme levels that officials issued environmental risk alerts, urging people to stay indoors. Sulfur dioxide (SO2): The NOM has three different limits to be met for concentrations of SO2: 8 hours (moving average), 24 hours (daily average) and annual average of the hourly data. During the last 10 years, concentrations below the three limits were recorded. However, in all stations the concentrations of this pollutant as a daily average are above the guideline value recommended by the World Health Organization, which is 0.008 ppm. From 2015 to 2017, the % of days with regular amounts of SO2 are minimal (above 5%). Nitrogen dioxide (NO2): The NOM defines that the maximum allowable limit is 0.210 ppm as an hourly average. To comply with the standard, the established limit must not be exceeded more than once a year. In all monitoring stations in Mexico City where this NOM was evaluated, concentrations below the limit were recorded during 2015 to 2017. However, this NOM has not been updated since 1993 and the World Health Organization recommends as a guide that the limit of the hourly average for NO2: be set to 0.106 ppm. Regarding the distribution of air quality by NO2, monitoring stations dominated the days with good air quality in Mexico City between 2015 and 2017. In each of the years more than 90% of the days presented good air quality. Carbon monoxide (CO): NOM establishes that the maximum permissible limit for CO is 11 ppm as a moving average of 8 hours. To comply with the standard, the established limit must not be exceeded more than once a year. Since 2011, the limit established in the NOM for CO in stations located within Mexico City has not been exceeded. Good air quality due to carbon monoxide predominate in all monitoring stations where it was possible to evaluate this pollutant. Environmental contingency On 16 May 2019, The government of Mexico City declared an extraordinary atmospheric environmental contingency, which indicates that the level of harmful particles in the air has exceeded the accepted standards (160 ppm for PM2.5). After a few hours, the same government would announce another "environmental contingency"